The present invention relates to a method of forming a coating film which can suitably be applied to articles to be coated having a complicated shape such as car bodies, and, more particularly, to a method of forming a coating film which comprises applying an electrodeposition coating [1] and then applying an electrodeposition coating [2], followed by baking and which enables application of a top coating without applying an intermediate coating, and to a coated article having the coating film obtained thereby.
The coating step of car bodies and the like is generally carried out by the so-called three-coat three-bake technique, namely by carrying out undercoating using a cationic electrodeposition coating, curing by baking, applying an intermediate coating, baking the intermediate coating film and, further, applying a base coating and a clear coating as a top coating and curing both coating films simultaneously.
Of these steps, the cationic electrodeposition coating employed as a means of undercoating is carried out mainly for the purpose of inhibiting corrosion and, therefore, even in the case of articles having a complicated structure, such as car bodies, it is necessary that all parts, in particular the inner panel site having a bag-like structure, be secured for sufficient corrosion inhibition. However, if an increased voltage is applied to form a sufficiently thick coating film until the inner panel site having a bag-like structure, the coating film on the outer panel portion will become excessive, whereby the coating amount will be unnecessarily increased and the finish appearance will be rather deteriorated. To avoid this problem, a sufficient degree of throwing power is to be secured. In this specification, throwing power means the successive formation of a coat on uncoated portion of the article to be coated.
A cationic electrodeposition coating excellent in throwing power, which comprises an ethynyl or nitrile group- or a like triple bond-containing fundamental resin within a molecule, is disclosed in WO 98/03701, for instance. In Japanese Kokai Publication 2000-38527, there is disclosed a cationic electrodeposition coating excellent in throwing power and shock resistance of coating films, which comprises an epoxy resin skeleton and has a sulfonium group, an aliphatic hydrocarbon group of 8 to 24 carbon atoms, which may optionally contain an unsaturated double bond within the chain thereof, and a propargyl group.
On the other hand, an intermediate coating generally occurs as a solution type coating and is applied by spraying. Since a good coating film appearance is required, the viscosity of the intermediate coating is strictly adjusted and, further, the spray coating must be carried out in an intermediate coating booth in which air conditioning, temperature adjustment and the like are made under highly controlled conditions. Thus, much cost and labor are required for the control.
So long as these undercoat coating film and intermediate coating film are concerned, the intermediate coating film is further required to have good weathering resistance, light degradation resistance, smoothness and so forth, while the undercoat coating film is required to have high corrosion prevention and rust prevention even at the bag-like structure portions. At the same time, resources saving and coating cost reduction are required from the environmental protection viewpoint.
In Japanese Kokai Publication Hei-09-125286, there is disclosed a coating method which comprises applying a thermosetting epoxy polyester resin-based powder coating to an article to be coated, carrying out baking to form an uncured coating film, then applying a thermosetting polyester-modified epoxy resin-based cationic electrodeposition coating to the portions not yet coated with the powder coating and curing the powder coating film and electrodeposited coating film simultaneously. This method, which comprises using the thermosetting epoxy polyester resin-based powder coating in lieu of the ordinary intermediate coating and then applying a thermosetting polyester-modified epoxy resin-based cationic electrodeposition coating to portions not yet coated with the powder coating such as the bag-like structure portions of the article to be coated, still requires the use of an intermediate coating booth, hence is not satisfactory from the coating cost reduction viewpoint. Furthermore, the vicinity of the interface between the outer panel site and bag-structured site of the article to be coated is coated in such a state that the thermosetting epoxy polyester resin-based powder coating is scattered, so that the film itself does not form a continuous layer. Thus, there arises the problem that even after the subsequent application of the thermosetting polyester-modified epoxy resin-based cationic electrodeposition coating, the rust prevention and finish are poor in the vicinity of the interface.
In Japanese Kokai Publication Hei-08-120494, there is disclosed a coating method which comprises applying a cationic electrodeposition coating mainly comprising a hydroxyl group-containing cationically electrodepositable vinyl copolymer, curing by heating, then applying a cationic electrodeposition coating mainly comprising a cationically electrodepositable epoxy resin, curing by heating and applying a water-borne base coating and a top coating. In Japanese Kokai Publication Hei-10-8291, there is disclosed a coating method which comprises applying a cationic electrodeposition coating capable of forming a coating film excellent in chipping resistance, preheating those portions having a film thickness less than the intended film thickness at 40 to 80xc2x0 C. and those portions having the intended film thickness at a temperature higher by 20 to 70xc2x0 C. as compared with those portions having a film thickness less than the intended film thickness and then applying a cationic electrodeposition coating mainly comprising an epoxy resin to thereby form a coating film on those portions having a film thickness less than the intended film thickness.
According to these methods, the ordinary coating step of an intermediate coating is omitted by applying two kinds of cationic electrodeposition coatings and the throwing power relative to the bag-structured portions is improved by heating for curing or preheating following application of the first stage cationic electrodeposition coating and thus preventing the second stage cationic electrodeposition coating film from depositing on the outer panel portions. However, in heating for curing or preheating following application of the first stage cationic electrodeposition coating, the temperature control is complicated, so that these methods are not satisfactory from the viewpoint of resources saving and/or coating cost reduction. Furthermore, in the vicinity of the interface between the outer panel site and the bag-structured site of the article to be coated, the coating film formed by applying the first stage cationic electrodeposition coating is very thin or the film itself occurs as a discontinuous layer, so that even after application of the second stage cationic electrodeposition coating, there remains the problem that the rust prevention of the vicinity of the interface is poor.
In view of the above-mentioned state of the art, it is an object of the present invention to provide a method of forming a coating film by which a coating film excellent in weathering resistance, light degradation resistance, smoothness and the like can be formed on the outer panel portion of an article to be coated such as a car, and a coating film excellent in rust prevention can be formed on the inner panel portion (bag-structured portion) of the article to be coated, with the interface between the outer and inner panel portions of the article being excellent in rust prevention and finish as well, and by which resources saving and coating cost reduction can be expected.
The present inventors found (1) that when a coating capable of providing performance characteristics required for the intermediate coating in the conventional use, namely weathering resistance, light degradation resistance, smoothness, chipping resistance, rust prevention (dry-wet method) and adhesiveness to materials and so forth, is used as a cationic electrodeposition coating in the first stage, the step of intermediate coating, followed by baking thereof, which is generally carried out, can be omitted and, thus, the process cost required for air conditioning, temperature adjustment and like controls can be reduced and the resources saving and coating cost reduction can be attained because of the reduction of the film thickness of the outer panel portion;
(2) that when an electrodeposition coating having a digital electrodepositability is used as the first stage cationic electrodeposition coating, the rust prevention and finish become good in the interface between the outer panel portion and the inner panel portion (bag-structured portion) of the article to be coated;
(3) that when a sulfonium group-containing resin is used in the first stage cationic electrodeposition coating, the step of heating for curing and the step of preheating between the first stage cationic electrodeposition coating and second stage cationic electrodeposition coating can be omitted; and
(4) that when an electrodeposition coating having high throwing power is used as the second stage cationic electrodeposition coating, a high level of corrosion prevention and rust prevention can be secured even in the inner panel portion (bag-structured portion) of the article to be coated and, further, only the inner panel portion (bag-structured portion) of the article to be coated is coated with the second stage cationic electrodeposition coating, so that the throwing power can be further improved. Based on these findings, the present invention has now been completed.
Thus, the present invention is related to a method of forming a coating film
which comprises applying an electrodeposition coating [1] to an article to be coated and applying an electrodeposition coating [2] thereon, followed by baking,
said electrodeposition coating [1] containing a sulfonium group-containing resin and
giving a film thickness to a face B of not more than one tenth of the film thickness of a face A when used in the electrodeposition coating of a coating with a resin solid matter of 20% by weight by a four sheet box method at 100 V and 40xc2x0 C. for 120 seconds following a rise time of 5 seconds to provide the face A with a 20 to 30 xcexcm film thickness and
said electrodeposition coating [2] having a time point at which the electric resistance value per unit volume of a deposited coat increases in the process of electrodeposition under a constant current condition.
The present invention is also directed to a coated article having the coating film formed by said method of forming a coating film.